This invention relates to an apparatus for continuously producing a heat-shrinkable crosslinked resin tube suitable for use in covering junctions in various pipe lines and cables as well as shielding various pipes and rods for corrosion prevention and heat insulation purposes. More particularly, it relates to an apparatus for continuously producing such a heat-shrinkable tube from a crosslinkable resin composition.
For the purposes of rust protection or heat insulation of junctions in lining steel pipes used in petroleum, gas, city water, chemical plant and other applications, or junctions in protective steel pipes used in electrical power cables and telecommunication cables, it has been a common practice to use a heat-shrinkable tube which can be shrunk by heating to cover the associated junction in a close sealing relationship. The heat-shrinkable tubes are tubes wherein molecular anisotropy is established by a structural change due to deformation of a heat-fusible polar chain high molecular weight compound and which utilize the phenomeon that the once established orientation is destroyed upon reheating to resume the original state. Various synthetic resins are used in the manufacture of such heat-shrinkable tubes and among them, crosslinkable synthetic resins such as crosslinkable polyethylene are now of most interest.
Continuous manufacture of such a heat-shrinkable crosslinked tube is known as described in Japanese Patent Publication No. 47-19356. The process uses a metal insert such as an aluminum pipe having a plurality of small perforations in the pipe wall. A crosslinkable resin compound is extrusion coated on the insert and the coated insert is then successively passed through crosslinking, expanding and cooling chambers whereby crosslinking is effected in the crosslinking chamber, and the resinous tube on the insert is then expanded by controlling the internal and external pressures of the tube in the expanding chamber. The tube in expanded form is cooled in the cooling chamber and then wound on a suitable take-up roll.
The above-mentioned prior art process uses a metal insert on which resin is extrusion coated. The use of a metal insert adds to the manufacturing cost. The metal insert must be finally removed from the surrounding resin tube, requiring a cumbersome operation. It is practically difficult or impossible to take up the tube in roll form without removing the metal insert. Therefore, the process is rather impracticable or commercially feasible with difficulty in the continuous manufacture of a heat-shrinkable tube.
In the prior art process, the resinous tube on the metal insert is expanded by controlling the pressures internal and external to the tube in the expanding chamber. The tube is kept free in the expanding chamber. No provision is made such that the outside surface of the expanded resin tube is positively supported. As a result, any slight unbalance between the internal and external pressures would cause excess or short expansion. The tube may crease in the cooling chamber as a result of an excessively increased diameter due to excessive expansion. This constitutes another factor of preventing the prior art process from being applied to commercial continuous manufacture of a heat-shrinkable tube.
A heat-shrinkable tube is generally fabricated from a crosslinkable resin composition by extruding the composition in a tubular form and continuously heating the tube in a crosslinking cylinder to effect crosslinking. The crosslinking cylinder commonly used is a cylinder whose interior wall surface is of such metal as stainless steel. The resin of the extruded tube may often be seized or adhered to the interior surface of the crosslinking cylinder. Lubricant oil is generally supplied between the extruded tube and the crosslinking cylinder inside surface for the purpose of reducing the friction therebetween to prevent resin seizing. Certain lubricant oils fail to provide lubrication because some constituents in the oil can react with the crosslinking agent in the resin composition, or some constituents in the oil can undesirably deteriorate the resin composition. The use of lubricant oil suffers from another problem that it can not be always fully distributed over the entire inside surface of the cross-linking cylinder, giving rise to lubricant losses which cause seizing. It is thus desired to develop a method capable of preventing seizing during crosslinking without resorting to the use of lubricant oil.
Generally, in the manufacture of a heat-shrinkable tube, heat shrinkability is imparted to the tube by expanding or increasing the diameter of the tube at a temperature of lower than the crosslinking temperature, but higher than the softening temperature at the end of crosslinking, and cooling the tube to a temperature sufficiently lower than the softening temperature while maintaining the diameter unchanged. When the prior art process as described above is practiced, the resin which has been heated hot in the crosslinking chamber remains still hot at a temperature near the crosslinking temperature upon entry to the expanding chamber. Such high temprature allows the resin to undergo crosslinking during the expanding step, failing to achieve sufficient heat shrinkability. Further, in the practice of the prior art process as described above, it is imperative that the cooling efficiency of the cooling chamber is reduced by the thermal influence of the expanding chamber. Then the cooling chamber must be extended to a sufficient length to increase the overall length of the apparatus, adding to the installation cost.
Since the heat-shrinkable tube is mainly used to cover various pipes and rods or junctions therein as previously described, an adhesive layer is often formed on the interior surface of the tube in order to ensure a bond of the tube to a subject upon covering. One known process for forming an adhesive layer on the heat-shrinkable tube interior surface for such a purpose is by separately applying adhesive to the interior surface of the once formed heat-shrinkable tube. Another approach employs a coextrusion technique in the manufacture of a heat-shrinkable tube wherein adhesive and resin compositions are coextruded to simultaneously form inside and outside layers. Either of these prior art processes for forming an adhesive layer on the heat-shrinkable tube interior surface suffers from many problems. The process of applying adhesive separately from the formation of a heat-shrinkable tube requires an increased number of steps which will add to the manufacturing cost. The two-coat simultaneous extrusion process for simultaneously forming a heat-shrinkable tube and an adhesive layer on the inside thereof relies on a special coextrusion technique requiring relatively expensive extruders and dies. Either of the prior art processes is unsuccessful in continuously producing a heat-shrinkable tube of a crosslinkable resin having an adhesive layer applied inside in an inexpensive efficient manner.
Therefore, a primary object of the present invention is to provide an improved apparatus for continuously producing a heat-shrinkable crosslinked tube in an efficient manner at low cost without incurring any problems as encountered in the prior art processes including the use of a metal insert.
Another object of the present invention is to provide an apparatus for consistently producing a heat-shrinkable crosslinked tube with a predetermined diameter in a steady manner.
A further object of the present invention is to provide an improved apparatus for continuously producing a heat-shrinkable crosslinked tube which can prevent seizing of the tubular resin in a crosslinking cylinder even in the absence of lubricant.
A still another object of the present invention is to provide an apparatus capable of consistently producing a heat-shrinkable crosslinked tube having improved heat stability and shrinkability, the apparatus being of a reduced length contributing to a reduction in overall installation cost.
A further object of the present invention is to provide an apparatus capable of continuously producing a heat-shrinkable tube of crosslinked resin having an adhesive layer applied inside in an inexpensive efficient manner.